Comparative study of soot formation on the centerline of axisymmetric laminar diffusion flames: Fuel and temperature effects

The appearance of soot on the centerline of axisymmetric laminar diffusion flames has been studied by monitoring (i) the gas temperature by thermocouples; (ii) the soot particle field by laser scattering/extinction; (iii) the presence of polycyclic aromatic hydrocarbons (PCAH) by laser induced fluorescence. Four fuels were used: butene, acetylene, butadiene, and benzene. All but one flame were at the smoke height condition and were characterized by different levels of N₂ dilution aimed at controlling the temperature field. It was observed that (i) soot nucleation occurs at the centerline; (ii) the soot onset on the centerline occurs when a characteristic temperature of 1350K is measured, regardless of fuel type or level of dilution; (iii) butene and benzene have similar fluorescence patterns, in contrast with premixed flame environments. These last two observations are consistent with the proposal that, though the extent of conversion of fuel into soot may significantly change from fuel to fuel, there is a common mechanism of soot formation for all fuels. The centerline fluorescence measurements indicate that the fluorescing species may contribute to the nucleation phase or, to a minimal extent, to surface growth in these diffusion flames. Time resolved measurements indicate that the fluorescence decay time is ≤ 10 ns, consistent with a possible identification of the fluorescing species with polycyclics such as acenaphthalene and pyrene.